1. Field of the Invention
This invention relates to a method for forming a color image and an ink to be used therefor, particularly to a method for forming, color images with extremely good lightfastness even when a mixed color portion is formed by overlapping inks of different colors on a recording medium, and according to which color images can also be obtained stably for a long time without any trouble such as clogging in an ink flow path such as a nozzle, etc., even in an ink jet system printer using heat energy, and an ink to be used therefor.
2. Related Background Art
In an ink jet system, recording is performed by forming ink droplets according to various ink ejecting systems, and then attaching some or all of them onto a recording medium such as paper, etc. As the ink used for forming color images according to such ink jet system, there may be employed an ink prepared by dissolving or dispersing water-soluble dyes of various hues, such as cyan, magenta, yellow, black, etc., in water or a liquid medium comprising water and a water-soluble solvent, and color images are formed using at least two inks with different hues.
Also, as the ink used for forming color images by use of writing implements such as a fountain pen, a felt tipped pen, ball point pen, etc., plural inks with different hues as described above have been used.
In the ink of the prior art as mentioned above, various performance characteristics are demanded as a matter of course, and among which the most frequently demanded characteristics concerns the lightfastness of the color images.
To elaborate on about this point, in an ink containing a dye, if the lightfastness of such dye is insufficient, there occurs frequently the problem that after image formation, the image will discolor or fade with time, whereby image quality is lowered.
Particularly, in the case of a color image, as the result of formation of the color image by superposition of different dyes of several kinds having hues on the paper, although individual dyes may exhibit excellent lightfastness, there ensues a new problem that lightfastness of the color image as a whole may be remarkably lowered by an interaction between the dyes.
For example, a copper phthalocyanine dye widely used as the dye for cyan ink in the ink jet system or a phthalocyanine dye as described in Japanese Laid-open patent publication No. 179569/1984 exhibits itself excellent lightfastness, but generally promotes fading of other dyes when superposed on such dyes on paper as described above; in particular it exercises remarkable influences on magenta dyes most and black dyes, particularly monoazo type magenta dyes. Accordingly, such problems must be solved very promptly in the present situation where the ink jet method for forming color images is attracting attention due to its advantages in handiness, cost, etc.
In formation of color images as described above, another important problem is liquid stability that the ink of any hue is free from clogging or generation of sediment matter at a nozzle, an orifice or a pen tip, during recording by use of the ink according to various systems, during intermission of recording and during periods of no recording for a long term.
Particularly among the ink jet systems, in the system employing heat energy, foreign matters are liable to be deposited on the surface of a thermal head due to temperature change, and therefore this problem is particularly important. Particularly, since plural inks are used when color images are formed, all of the inks are required to have sufficient liquid stability.
However, in the case of the inks of the prior art, some additives are required to be used for satisfying various conditions such as ejecting condition of ink, storage stability for a long term, sharpness and density of images in recording, surface tension, electrical properties, etc., and also due to various impurities contained in the dyes used, there have been various problems such that clogging occurred at a nozzle or an orifice of an ink jet device or deposited matter was generated on the surface of the heating head, or sediment matter was generated during storage for a long term. Thus, in spite of various excellent characteristics of the ink jet system, this is one reason why it has not attained widespread popularity.